Definition of "Dual" :
adjective: a grammatical number category referring to two items or units as opposed to one item (singular) or more than two items (plural)
"Ancient Greek had the dual form but it has merged with the plural form in modern Greek."
adjective: consisting of or involving two parts or components usually in pairs
"Dual controls for pilot and copilot."
adjective: having more than one decidedly dissimilar aspects or qualities
"A double (or dual) role for an actor."